1. Field of Invention
The present invention is directed to controllers for electroluminescent lamps. More specifically, the invention involves electronic control circuits that sense and enhance the viewing contrast of various sized cells within electroluminescent (EL) lamps while these cells are dynamically switched.
2. Discussion of Related Art
EL lamps provide solutions to many lighting problems that require very thin yet rugged structures, which are light and flexible and have an infinite number of sizes and shapes. EL lamps are also fairly inexpensive and simple to construct. The controllers required for EL lamps must obtain a lamp brightness that allows a viewer the ability to distinguish the shapes and information presented by the lamp""s design. The maintenance and control of this brightness becomes more complicated when considering the large voltages involved and variables such as changing ambient lighting, switching between cells of different sizes, extending the limited lifetimes of EL lamps, and operating within the limitations imposed by the lamp construction.
EL lamps are also subject to aging effects that vary the brightness of the lamps"" output even when driven with a constant amplitude and frequency drive signal. EL lamps are capacitive in nature, and the capacitance of the EL lamp varies with the operating age of the EL lamp (i.e., the time during which the EL lamp has been operated). In addition, the surface area of the EL lamp is a determinant of the amount of capacitance. Construction techniques of various manufacturers are a further determinant of the initial amount of capacitance exhibited by an EL lamp.
Inventors have created controllers that supply the large voltages necessary to drive EL lamps and some have added compensation techniques that improve visibility. However, U.S. Pat. Nos. 5,519,288 and 5,517,089 sense only the output voltage to the EL lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,440,208 senses only the current flowing through the EL lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,336,978 senses the light output of a single cell, which does not provide proper compensation concerning the issues of changing ambient light nor switched-multiple-sized cells. U.S. Pat. No. 5,293,098 does not sense any of the output conditions of the lamp, thus changing ambient light, switched-multiple-sized cells and aging are not properly compensated. U.S. Pat. No. 5,144,203 delivers only constant power to EL lamp which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,089,748 senses the light output of a single lamp which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells. U.S. Pat. No. 4,443,741 senses the current of a single lamp, which does not provide proper compensation for the issues of changing ambient light and switched-multiple-sized cells. U.S. Pat. No. 4,319,165 adjusts the power factor of a single lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging.
All controllers known heretofore suffer from a number of disadvantages. Firstly, changing ambient light dramatically decreases the viewer""s ability to distinguish information being presented by the lamp. Unfortunately manual adjustments of luminance to compensate for changing ambient light conditions require constant intervention. In addition, high luminance settings can significantly reduce the lifetime of the battery in battery operated EL lamps, while low settings make the lamps ineffective. High luminance settings can also reduce the lifetime of lamps. Presently known controllers cannot adjust luminance when switching between cells of different sizes meaning that large luminance variations and transients result when dynamically switching EL cells within lamps. Proper compensation for EL lamp aging for various sized cells being sequenced within an EL lamp does not exist in the prior art. Further, presently known lamp controllers cannot vary or set the nominal luminance. Finally, presently known lamp controllers can not shut down inverters when lamps are disconnected, resulting in large AC voltages appearing at the controller outputs.
In view of the above disadvantages of the prior art, it is an object of the present invention to provide an ambient sensor that automatically corrects for environmental changes in light directed onto the lamp, making lamp information readable, allowing lamps to age more slowly, and allowing battery operated systems to last longer. It is also an object of the present invention to provide an EL lamp system which operates independently of the aging effects of its EL elements. It is another object of the present invention to provide an EL lamp system which compensates for the age of its EL elements. It is a further object of the present invention to provide adjustable gains in the ambient feedback loop so that a controlled maximum increase in lamp luminance can be selected. It is another object of the present invention to provide appropriate feedback time constants that allow response time of the corrections to track desired changes. It is a still further object of the present invention to provide an automatic ambient control that integrates with selectable settings for lamp luminance, aging in the lamps, and dynamic switching of various sized cells within the lamp. It is yet another object of the present invention to provide frequency and voltage compensation that automatically adjusts the luminance quickly for applications requiring sequencing cells of different sizes. It is still another object of the present invention to provide constant cell-to-cell and long-term luminance to extend lamp lifetime while a lamp is being sequenced through cells of various sizes. It is a further object of the present invention to provide manual luminance control that is independent of AC line input variations and allow for accurate and repeatable levels of luminance for lamps of various constructions.
Further objects and advantages include providing a controller which removes many constraints on lamp designs. This approach can also eliminate many quality control problems with lamp construction. The controller can shut down the inverter whenever the lamp is disconnected, and can shut down the inverter when excessive loads or overheating occur. This approach widens the operational range for lamps and cell sizes. Still further objects and advantages will become evident with the following descriptions and drawings.
The above objects are each achieved according to an aspect of the present invention by providing electronic sensing circuits which monitor the multiple-sized electroluminescent cells of the lamp and make adjustments to compensate for variations in the contrast between the cells, while maintaining a fixed contrast between the lamp luminance and the ambient lighting. Separate feedback loops monitor the ambient lighting, the cell luminance and the frequency of the excitation voltage and make appropriate adjustments to an adjustable luminance reference. The adjustments occur while a microprocessor sequences through an assortment of electroluminescent cells of various sizes. In another embodiment, instead of having a feedback loop to monitor the lamp luminance as it decreases due to aging, the circuit which drives the EL lamp includes a timer and a microprocessor. The timer measures the elapsed time during which the EL display has been operating. The microprocessor adjusts the drive signal to the EL display to compensate its brightness to be independent of its age, based on the elapsed time measured by the timer and an empirically-determined aging parameter.